1. Technical Field
The present invention relates to an image capturing element, an image capturing apparatus, an image capturing method and an image capturing system capable of reading charge from photoelectric conversion elements by a destructive readout method and a non-destructive readout method and to an image processing apparatus capable of generating captured image data on the basis of charge read by a destructive readout method and a non-destructive readout method.
2. Related Art
In the related art, a destructive readout method and a non-destructive readout method have been used to read out charge from image sensors (photoelectric conversion elements) of an image capturing apparatus.
The destructive readout method includes a reset process of clearing charge stored in the photoelectric conversion element when reading the charge (pixel signal) from the photoelectric conversion element.
The non-destructive readout method reads out the charge (pixel signal) from the photoelectric conversion element without clearing the charge stored in the photoelectric conversion element. That is, the non-destructive readout method does not perform the reset process when reading the charge. Therefore, in the non-destructive readout method, it is possible to read charge at different exposure times again and again until exposure time reaches a set value while the charge is being stored in the photoelectric conversion element. Thus, the non-destructive readout method makes it possible to easily realize multistage exposure.
Further, in the related art, the following monitoring method using an outdoor surveillance camera has been proposed: a light source is provided around the camera and radiates light to a specific region, which makes it possible for a guard to monitor whether an object is entered into specific region. This method is similar to a method of capturing the image of a person in the dark place using an electronic flash. That is, when light emitted from a light source is reflected from a subject, the camera receives the reflected light and records the image of the subject. Meanwhile, since flashlight does not reach a distant dark background, it is necessary to lengthen the exposure time to capture the image of the distant background, and the background image is recorded on the camera.
In the above-mentioned surveillance camera, it is preferable to capture the image of a distant background and a specific region (near object). In this case, a wide dynamic lens camera (image capturing element) needs to be used to capture an image having high contrast. A non-destructive image capturing element has been proposed as an image capturing element for realizing the wide dynamic lens camera. An image capturing apparatus disclosed in JP-A-5-22670 and a solid-state image capturing element disclosed in JP-A-7-38815 have been known as the non-destructive image capturing element having the wide dynamic lens.
The image capturing apparatus disclosed in JP-A-5-22670 includes a plurality of pixels each having an amplification-type photoelectric conversion element, and reads a plurality of pixel signals having different exposure times. When a saturation pixel signal is detected from one pixel signal having long exposure time, the saturation pixel signal is replaced with a non-saturation pixel signal of the pixel having short exposure time, thereby realizing a wide dynamic lens.
The solid-state image capturing element disclosed in JP-A-7-38815 is provided with a vertical scanning circuit that outputs first and second read signals having different timings within a horizontal effective period to a light receiving unit having photoelectric conversion elements as pixels arranged in a matrix in plan view and a horizontal scanning circuit that inputs a start signal two times within the horizontal effective period. The two reading operations are performed with the horizontal scanning period at a predetermined interval, and pixel signals are independently read out without providing a plurality of vertical signals lines, thereby realizing a wide dynamic lens.
The techniques disclosed in JP-A-5-22670 and JP-A-7-38815 can realize a wide dynamic lens. However, for example, when a subject to be photographed moves relative to a dark background and thus a frame rate (exposure time) is matched with the moving subject, the background appears to be dark in the captured image, which makes it difficult to recognize the content of the background. On the other hand, when image capturing is performed with the frame rate being matched with the background, the frame rate (exposure time) increases and motion blur occurs in the captured image, which makes it difficult to determine the movement of an object to be monitored.